1. Field of the Invention
The present invention is related to viewing optics, and in particular, a lightweight head mounted display (HMD) system combining aspheric and diffractive characteristics in a single element.
2. Description of the Related Art
In recent years, many head-mounted displays have been designed and produced for the virtual reality marketplace and other applications. Many of these systems are large in size, heavy, bulky, and exhibit poor optical performance. Imagery presented to the user can be fuzzy over all or some of the field of view. Annoying lateral color (i.e., color fringing) around objects and image distortion can occur.
Most prior art viewing optics design forms consist of from one to a plurality of lens elements used to magnify the object being viewed. These optical systems provide the user with a large apparent field of view with a comfortable and large eye relief clearance to the eye. These systems attempt to provide high image quality with minimal residual image degrading aberrations, such as astigmatism, coma, field curvature, lateral color, and distortion. The optical aperture stop of these systems is the iris of the human eye, and therefore is remote from the lens system itself. Some of the well-known methods of eliminating aberrations, such as design symmetry in many photographic camera lenses, thus cannot be used. For this reason, virtually all prior art viewing optics designs have severe image degrading aberrations.
Most prior art viewing optics designs are derived from eyepieces that were designed for telescope or microscope applications. These designs most often use glass lenses, and almost always have severe image degrading aberrations, especially in the outer periphery of the field of view. For example, prior art viewing optics such as those disclosed in U.S. Pat. Nos. 5,587,837, 5,557,463, 4,525,035, 4,482,217, 4,099,842, and 3,390,935, could exhibit significant residual off axis aberrations including, in particular, astigmatism, coma, lateral color, and distortion. While these patents are representatives of the prior art in viewing optics design, there are many similar prior art patents that also exhibit such limited optical performance.
Some of the design approaches use more optical elements to reduce image degrading aberrations. For example, U.S. Pat. No. 3,612,662 contains 10 lens elements. But, performance is degraded because of blurring at the outer periphery of the field of view. Using more optical elements also is disadvantageous because of the increased size, weight, and cost.
Other new design approaches use one or more plastic lens elements to reduce production costs and allow for non-spherical surfaces to reduce image-degrading aberrations. Using plastic elements that weigh less than glass also results in an overall weight reduction. While this approach is partially successful, lateral color and some other off-axis aberrations cannot be eliminated adequately.
Some of these new design approaches attempt to remedy such limitations. For example, U.S. Pat. No. 5,790,312 discloses a novel viewing optics design that is compact, light-weight, and of high performance. Special manufacturing methods, however, are required, and the design is not suitable for all applications. Further, due to inherent asymmetry in the design, the residual aberrations are asymmetrical with field position, which is disadvantageous.
Another design approach, as in U.S. Pat. No. 5,148,314, uses diffractive optics to supplement conventional refractive lenses. While such diffractive/refractive surface combinations permit correction of lateral color through dispersion compensation, and correction of other chromatic and monochromatic aberrations, most of these designs suffer from poor diffraction efficiency, which reduces contrast in resulting imagery.
All of the referenced prior art designs exhibit image degrading problems to varying degrees. And virtually all prior art designs are optimized on a flat imaging field. Very often a balance of aberrations, including astigmatism and coma, are used to maximize the optical performance. Unfortunately, this still often results in residual aberrations. Thus, there is a need for viewing optics with improved optical performance.
In general, in one aspect, embodiments of the invention feature an imaging optical system having a plurality of optical elements. The imaging optical system includes an aspheric kinoform diffractive element having an aspheric surface profile and a superimposed diffractive kinoform profile. The aspheric kinoform diffractive element is adapted to minimize residual optical aberrations and produce an optimized image on a curved image field of the imaging optical system.
Embodiments of the present invention offers advantages over prior art viewing optics in head mounted display systems and other applications requiring a close visually-viewed and magnified image of an object. These optical systems are often used as eyepieces, and their design forms evolved from the eyepieces used in telescopes, microscopes, and other visual optical systems.
Embodiments of the present invention feature a viewing optics design form that overcomes some or all of the deficiencies of prior art viewing optics designs by using the element having the diffractive surface superimposed on an otherwise nearly flat aspheric surface profile. This surface is positioned in the viewing optics where all ray angles incident on it are small and nearly the same over the whole field of view.
Allowing the design to be optimized on a slightly curved field or image surface provides a further advantage, resulting in improved optical performance over prior art designs having flat image fields. The curved field allows the elements in the system to control image blurring aberrations rather than having to also control field curvature as in prior art systems. The human eye can accommodate in its dioptric power for the curved field, and most users have no problem seeing the entire field in focus. Thus, the curved field does not reduce image quality, but instead allows image quality to be enhanced.
A technique often used in eyepieces to correct field curvature is to place a field flattening lens close to the image plane (or in an HMD, the imager plane). In an HMD application using a reflective imager, this is not possible due to space requirements for illumination injection. Thus, prior art designs that are optimized on a flat plane generally end up with residual field curvature partially balanced by astigmatism. Embodiments of the present invention feature the aspheric kinoform diffractive element to avoid or reduce this problem. A further advantage over the prior art is that the diffractive surface has a large kinoform pitch, which makes it significantly more manufacturable than prior art diffractive designs.
Embodiments of the imaging optical system further feature a cemented doublet and, in some embodiments, a flat or nearly flat faceplate that can be used, for example, with a display device such as a reflective liquid crystal display (LCD). The aspheric element can be made of, for example, acrylic. These embodiments reduce chromatic aberrations and minimize most of the monochromatic aberrations of the imaging optical system.
Embodiments of the present invention also feature use for image projection, in which case, instead of a user viewing an image through the imaging optical system, image light is projected for viewing onto a screen.